currently kmail won't allow grouping by email title/subject. if it did, any replies to this bug could be grouped together in kmail :)
You already tried View / Message List / Sorting / By Subject ?
that sorts by subject but there is no "gouping/threading" by subject.
I was thinking maybe it could group emails with the same subject the same way it does with emails from the same thread in a mailing list.
Basically I would like to a variation of "Standard mailing list" aggregation that works even if I don't have the initial message. The current form doesn't do aggregation unless I have the initial message or if this is an actual mailing list.
For example, I would like all mail titled "[kmail2] [Bug 319713] allow grouping by title (topic/subject)" to be in the same thread. Kmail doesn't do that right now unless the first email is from me (meaning I reported the bug)